This invention relates to an improved process for exhaust dyeing nylon textiles.
Nylon fiber is commonly dyed with acid dyes, including premetallized acid dyes, in a batch process referred to as exhaust dyeing. For example, nylon fiber which has been made into fabric may be dyed in a jet-dyeing machine, whereby a continuous loop of the fabric is circulated throughout the dye bath by impinging the dye bath liquor against the fabric in a venturi nozzle.
Care must be taken during the dyeing process to obtain a uniform distribution of dye on the fabric, referred to as leveling. One method of improving leveling is to incorporate leveling agents which retard the dye uptake. Another method is to gradually raise the temperature of the dye bath by about 1.degree. to 2.degree. F. per minute from a starting temperature of approximately 90.degree. F. to a dye temperature of approximately 200.degree. F. The rise in temperature causes the fiber structure to open up and increases diffusion and migration of the dye.
The pH of the dye bath is a critical factor in dyeing nylon with acid dyes and has a profound effect on the dye strike rate. At low pH, the acid functionality of the dye reacts with the activated amino groups of the polyamide, to form a salt. Generally, the pH at the beginning of the dye cycle is neutral and is slowly decreased during the dye cycle. The pH of the dye bath is controlled by one or more of the following techniques: use of buffers; gradual addition of mild acids, such as acetic acid during the heating cycle; and use of diesters which hydrolyze in the presence of water, especially at higher temperatures, to form an acid.
It is desirable to lower the pH to 3.5 or less during the dye cycle to maximize dye exhaustion. However, it is very difficult to achieve pH levels below 4.0 without losing control of the dye strike rate, thereby causing variations in dye coverage on the fabric. The pH control is especially necessary with premetallized dyes because of their greater reactivity. While the use of mild acids and diesters is successful in achieving level dyeing, they are not effective in lowering the pH much below 4.5.
Therefore, while the objectives of uniform dye application on fabric has been met by the aforementioned methods, the degree of dye exhaustion has not been satisfactory. Incomplete exhaustion of the dye is reflected in increased raw material costs, since excess dye must be used in each cycle; greater rinsing to remove residual, unreacted dye; and a burden placed on waste water treatment systems to dispose of unreacted dyes.